Gas absorption spectroscopy measures the concentration of a species of gas of interest in a gas sample by passing an electromagnetic signal through the sample and detecting the absorption at the wavelength of a spectral absorption feature of the species of interest. A spectral feature is an absorption line representing a frequency of electromagnetic radiation corresponding to a vibrational, rotational or electronic transition of a molecule of the gas. Tunable diode lasers are ideal for absorption spectroscopy since these lasers can be tuned to the center of a spectral feature and these lasers provide a narrow signal relative to the width of the spectral feature. Temperature adjustment provides coarse tuning and DC current adjustment provides fine tuning of the laser diode to a frequency near the center of the spectral feature.
In Frequency Modulation Spectroscopy (FMS) a small AC signal is superimposed on top of the DC current of the diode laser to modulate the frequency of the laser beam across the center of the spectral feature. The modulated laser beam is passed through a sample of gas to a photodetector that measures the intensity of the laser beam. Absorption is greatest at the center of the spectral feature and absorption reduces as the frequency of the laser sweeps away from the center. As the laser frequency is modulated across the center, a periodic signal is produced by the photodetector. This resulting signal is expanded in a Fourier Cosine Series, the coefficients of expansion being denoted as harmonics. The fundamental or first harmonic is analyzed to monitor the transmitted laser power. The even harmonics exhibit maxima at the linecenter of the spectral feature and the second harmonic is analyzed to compute the concentration of the gas of interest.
The initial tuning of the laser results in the laser projecting a beam with a frequency near the center of the spectral feature. After initial tuning, the frequency of the laser beam can drift due to temperature and current variation. Line-locking the laser means preventing frequency drift and maintaining the modulated frequency of the laser beam centered at the desired frequency. The odd harmonics exhibit zero crossings at the center of the spectral feature. In conventional FMS systems the center zero crossing of the third harmonic is monitored to provide closed-loop control of the laser to line-lock the laser to the center of the spectral feature. FMS provides fast, accurate measurement and can detect trace gases in a sample.
If the gas of interest is difficult to contain in a reference cell, such as a highly corrosive gas or a short lived gas, the diode laser cannot be line-locked by monitoring the third harmonic of the gas of interest in a reference cell. In U.S. Pat. No. 5,459,574 to Lee et al., a tunable diode laser is first line-locked to the frequency of a spectral feature of a first gas in a reference cell and then the frequency is displaced a predetermined amount to the frequency of a spectral feature of a second gas to measure concentration of the second gas. The laser is free running and not line-locked during the measurement portion of the cycle. The laser is not line-locked to the second frequency and can only be maintained at this second frequency for a limited period of time.